Now, I can see where you would get confused...

Picture this: Customer X calls up, wanting an additional domain authorised to service Y. Fair cop, easy job. All I need to do is to send an authorisation email to address Z on said domain, and all the customer needs to do is to reply back to the system from address Z. The authorisation will only be verified if the reply comes back from that address. Seems easy enough, right? I explained this pretty clearly to the customer and, to his credit, the customer seemed to follow. So I send the authorisation email off to address Z, and end the call.

...Three minutes later, I get a system generated email back, saying that authorisation was refused. Why, I ask?

Authorisation was rejected as the addresses are mismatched.

Sent to "Address Z"
Received From "Address A"

...

...Oh, for fuck's sake...

So I've just sent the customer an email (to address Z, A, and 17), explain that I had to resend the authorisation email, WHY I had to resend the email, and how to do it properly this time.

And a paraphrased "...this time, try listening the fuck up".